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Peach Mountain Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Peach Mountain Observatory clear sky chart from 2005-11-06 to 2024-01-16.

This page is a summary of past forecasts. Even though it's based only on forecasts, some people use these pages as clue to a site's climate. But climate data based on actual observation should be preferred (if you can find it!).

Cloud, Transparency and Seeing forecasts are summarized individually. But because good observing conditions require a combination of minimum cloud cover, transparency and seeing, the individual forecast summaries can be difficult to interpret. The observing conditions section counts the combinations of conditions needed for common types of astronomical observation.

Observing Conditions

The following tables count the occurrence of the combinations of forecast and darkness conditions needed for different observations. Conditions are rated by by what percentage of the hours the observation would likely be attempted are usable.

Observation Type	Minimum conditions for hour to be usable				Out of hours
	Clear of Clouds	Transparency	Seeing	Sky Brightness
Planetary:	50%	(no limit)	Average	(no limit)	Nighttime Hours
Deep Sky:	70%	Average	(no limit)	5.9	Nighttime Hours
CCD Imaging:	70%	Below Average	Poor	(no limit)	Nighttime Hours

Percentages are of nighttime hours per month, or per year, when a particular observing conditions was met. Deep Sky percentages never exceed 50% because the moon is up half the time. Deep sky percentages are much lower for charts at high latitudes because of fewer dark hours in the summer.

Overall

By Month

Individual Forecasts

The length of each colored bar in the "occurrence of each Forecast" column is proportional to the number of times the chart forecast that color. The percentages are of the total number of hours per month, or per year, with a particular forecast. The colors as the same as those used in clear sky charts.

Note: Transparency probabilities are only counted for those hours that have 30% or less cloud cover. Seeing probabilities are only counted for those hours that have 80% or less cloud cover. If that's too tricky, try Observing Conditions instead.

Overall

		Occurrence of each Forecast	Data Points
Cloud Cover	Day	14% 9% 4 7% 6% 4 5% 5% 5% 7% 33%	74654
	Night	15% 10% 4 7% 6% 3 5% 4 5% 7% 34%	75131
Transparency	Day	49% 7% 12% 20% 12% 1	74363
	Night	49% 6% 12% 20% 12% 1	74943
Seeing	Day	18% 11% 29% 23% 14% 6%	14412
	Night	19% 9% 29% 29% 12% 1	24493

By Month

Percentage Cloud Cover SE (2006-)
Color Meanings: Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	16% 15% 6% 11% 7% 5% 5% 5% 4 6% 20%	1517
	11th to 20th	Day	16% 17% 7% 10% 9% 4 6% 5% 4 5% 16%	1480
	21st to end	Day	17% 18% 8% 10% 8% 3 4 4 4 6% 18%	1711
February	1st to 10th	Day	15% 14% 6% 11% 10% 3 6% 4 5% 6% 19%	1620
	11th to 20th	Day	15% 11% 5% 12% 10% 3 5% 3 5% 6% 24%	1728
	21st to end	Day	16% 15% 5% 11% 6% 3 5% 3 4 6% 25%	1480
March	1st to 10th	Day	15% 11% 5% 7% 7% 4 5% 6% 7% 6% 27%	1818
	11th to 20th	Day	13% 12% 4 8% 6% 3 4 4 5% 6% 34%	1980
	21st to end	Day	20% 12% 4 8% 6% 3 4 4 4 7% 29%	2178
April	1st to 10th	Day	17% 11% 5% 6% 6% 3 4 5% 5% 7% 30%	2124
	11th to 20th	Day	17% 9% 4 7% 5% 4 4 5% 5% 7% 34%	2253
	21st to end	Day	17% 10% 4 7% 6% 3 4 3 5% 7% 35%	2340
May	1st to 10th	Day	17% 8% 3 7% 6% 3 5% 5% 6% 7% 32%	2340
	11th to 20th	Day	18% 8% 3 6% 6% 3 5% 4 5% 6% 35%	2301
	21st to end	Day	12% 7% 2 5% 5% 4 5% 5% 6% 9% 40%	2557
June	1st to 10th	Day	10% 5% 3 5% 5% 3 6% 5% 6% 9% 42%	2520
	11th to 20th	Day	11% 6% 3 5% 5% 4 6% 5% 6% 9% 40%	2520
	21st to end	Day	9% 5% 3 6% 5% 4 6% 6% 7% 10% 39%	2520
July	1st to 10th	Day	7% 5% 3 4 4 3 6% 5% 7% 10% 46%	2520
	11th to 20th	Day	9% 5% 3 5% 5% 3 6% 5% 6% 9% 44%	2520
	21st to end	Day	7% 5% 2 5% 6% 4 5% 6% 7% 9% 42%	2627
August	1st to 10th	Day	8% 5% 2 6% 6% 4 6% 6% 7% 10% 41%	2327
	11th to 20th	Day	10% 6% 3 6% 6% 5% 6% 5% 7% 8% 38%	2303
	21st to end	Day	9% 6% 3 5% 6% 3 5% 5% 6% 9% 43%	2512
September	1st to 10th	Day	11% 6% 2 6% 7% 4 5% 5% 5% 8% 40%	2057
	11th to 20th	Day	10% 5% 3 4 5% 3 5% 5% 6% 8% 46%	1980
	21st to end	Day	12% 6% 3 6% 5% 4 5% 5% 6% 8% 39%	1980
October	1st to 10th	Day	13% 7% 3 6% 6% 3 5% 5% 5% 6% 41%	1954
	11th to 20th	Day	18% 8% 4 8% 7% 4 5% 4 5% 6% 32%	1800
	21st to end	Day	21% 10% 4 7% 6% 3 4 4 4 7% 30%	1805
November	1st to 10th	Day	12% 9% 4 7% 6% 3 4 5% 5% 6% 39%	1672
	11th to 20th	Day	15% 13% 5% 8% 6% 4 5% 4 5% 6% 30%	1710
	21st to end	Day	20% 11% 5% 6% 6% 3 5% 5% 5% 6% 28%	1710
December	1st to 10th	Day	13% 15% 8% 11% 8% 4 6% 3 4 6% 21%	1543
	11th to 20th	Day	15% 15% 7% 10% 6% 4 5% 6% 5% 7% 21%	1864
	21st to end	Day	25% 20% 8% 10% 7% 3 4 4 3 4 11%	2783
January	1st to 10th	Night	18% 12% 6% 10% 8% 4 4 4 5% 5% 24%	2648
	11th to 20th	Night	19% 15% 6% 10% 7% 4 5% 4 5% 6% 20%	2591
	21st to end	Night	13% 14% 9% 12% 8% 4 5% 4 5% 6% 21%	2642
February	1st to 10th	Night	18% 15% 6% 9% 9% 3 6% 4 4 5% 22%	2340
	11th to 20th	Night	15% 13% 6% 9% 8% 3 5% 4 5% 6% 27%	2116
	21st to end	Night	19% 11% 5% 10% 6% 3 4 4 5% 6% 29%	1776
March	1st to 10th	Night	16% 10% 5% 7% 7% 3 4 5% 4 5% 34%	2142
	11th to 20th	Night	13% 10% 3 6% 5% 3 5% 5% 6% 8% 37%	1980
	21st to end	Night	20% 9% 4 6% 6% 3 5% 4 5% 6% 33%	2178
April	1st to 10th	Night	15% 8% 4 6% 7% 3 5% 5% 4 7% 36%	1850
	11th to 20th	Night	18% 9% 2 4 5% 3 5% 5% 5% 7% 36%	1654
	21st to end	Night	17% 9% 3 5% 5% 3 5% 5% 4 6% 38%	1620
May	1st to 10th	Night	17% 8% 3 7% 7% 3 6% 5% 5% 8% 32%	1620
	11th to 20th	Night	19% 6% 2 5% 5% 3 4 5% 5% 7% 39%	1592
	21st to end	Night	10% 6% 2 5% 5% 3 6% 4 6% 9% 44%	1713
June	1st to 10th	Night	10% 5% 2 4 5% 3 5% 4 5% 8% 48%	1440
	11th to 20th	Night	11% 5% 2 5% 5% 4 4 5% 5% 9% 44%	1440
	21st to end	Night	9% 5% 3 6% 4 2 4 4 5% 11% 47%	1440
July	1st to 10th	Night	6% 4 2 5% 5% 4 5% 4 5% 8% 52%	1440
	11th to 20th	Night	8% 4 2 5% 4 2 3 4 6% 9% 52%	1440
	21st to end	Night	4 3 2 4 5% 3 5% 5% 7% 10% 51%	1691
August	1st to 10th	Night	7% 4 2 5% 4 3 5% 4 5% 10% 50%	1611
	11th to 20th	Night	8% 5% 2 5% 5% 3 4 6% 6% 8% 49%	1598
	21st to end	Night	8% 5% 2 4 5% 3 4 5% 5% 7% 52%	1840
September	1st to 10th	Night	10% 6% 2 5% 6% 3 4 5% 5% 7% 48%	1885
	11th to 20th	Night	10% 5% 2 4 4 2 4 3 5% 7% 53%	1980
	21st to end	Night	13% 5% 3 5% 5% 3 4 5% 5% 6% 44%	1980
October	1st to 10th	Night	13% 7% 3 6% 5% 3 5% 4 4 6% 45%	2004
	11th to 20th	Night	19% 9% 4 8% 6% 3 4 4 5% 6% 34%	2160
	21st to end	Night	22% 9% 3 6% 5% 3 5% 4 5% 6% 33%	2546
November	1st to 10th	Night	13% 9% 3 6% 5% 3 4 4 5% 6% 43%	2406
	11th to 20th	Night	16% 10% 4 7% 6% 4 5% 5% 6% 7% 31%	2470
	21st to end	Night	19% 13% 5% 7% 6% 3 4 4 4 5% 30%	2470
December	1st to 10th	Night	16% 15% 7% 10% 8% 4 4 4 4 6% 21%	2626
	11th to 20th	Night	15% 13% 5% 11% 6% 4 6% 5% 5% 6% 23%	3318
	21st to end	Night	27% 22% 6% 9% 6% 3 4 4 4 4 12%	4884

Transparency (when cloud cover <= 30%)
Color Meanings: Too cloudy to forecast Poor Below Average Average Above average Transparent
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	64% 6% 11% 13% 6%	1518
	11th to 20th	Day	68% 8% 11% 8% 4	1480
	21st to end	Day	69% 4 9% 12% 5% 1	1711
February	1st to 10th	Day	65% 6% 11% 12% 6%	1620
	11th to 20th	Day	63% 5% 9% 14% 8% 1	1738
	21st to end	Day	61% 5% 11% 15% 7% 1	1480
March	1st to 10th	Day	55% 8% 11% 16% 8% 1	1818
	11th to 20th	Day	51% 5% 12% 18% 12% 3	1980
	21st to end	Day	57% 5% 10% 19% 8% 1	2178
April	1st to 10th	Day	54% 5% 10% 19% 11%	2124
	11th to 20th	Day	49% 6% 11% 19% 13% 2	2253
	21st to end	Day	50% 6% 13% 16% 14% 1	2340
May	1st to 10th	Day	49% 8% 12% 19% 10% 2	2340
	11th to 20th	Day	49% 7% 10% 18% 14% 2	2305
	21st to end	Day	40% 7% 14% 24% 13% 2	2557
June	1st to 10th	Day	37% 7% 12% 27% 14% 2	2520
	11th to 20th	Day	39% 8% 12% 23% 17% 2	2520
	21st to end	Day	38% 9% 14% 22% 16% 2	2520
July	1st to 10th	Day	33% 8% 12% 25% 19% 4	2451
	11th to 20th	Day	35% 8% 13% 24% 18% 2	2445
	21st to end	Day	35% 9% 14% 23% 18% 2	2481
August	1st to 10th	Day	35% 10% 14% 27% 13% 1	2314
	11th to 20th	Day	41% 9% 12% 23% 14% 2	2303
	21st to end	Day	37% 7% 11% 25% 18% 2	2512
September	1st to 10th	Day	41% 8% 15% 21% 14% 2	2057
	11th to 20th	Day	34% 7% 12% 32% 14% 1	1980
	21st to end	Day	41% 8% 12% 25% 14%	1980
October	1st to 10th	Day	42% 8% 11% 24% 14% 2	1954
	11th to 20th	Day	52% 6% 12% 19% 11%	1800
	21st to end	Day	55% 5% 13% 17% 9%	1805
November	1st to 10th	Day	44% 6% 13% 22% 14% 1	1672
	11th to 20th	Day	55% 5% 12% 19% 8% 2	1710
	21st to end	Day	56% 6% 13% 17% 7%	1710
December	1st to 10th	Day	65% 5% 11% 15% 4	1543
	11th to 20th	Day	61% 7% 14% 14% 4	1861
	21st to end	Day	78% 5% 8% 6% 3	2783
January	1st to 10th	Night	62% 5% 10% 16% 6%	2648
	11th to 20th	Night	66% 5% 14% 10% 6%	2591
	21st to end	Night	64% 6% 12% 12% 6% 1	2642
February	1st to 10th	Night	64% 7% 9% 13% 5% 1	2340
	11th to 20th	Night	58% 5% 10% 16% 9% 1	2117
	21st to end	Night	56% 4 13% 17% 8% 2	1776
March	1st to 10th	Night	51% 5% 11% 21% 10% 1	2142
	11th to 20th	Night	45% 6% 11% 20% 15% 2	1980
	21st to end	Night	52% 4 10% 21% 11% 2	2178
April	1st to 10th	Night	47% 6% 11% 21% 14% 1	1850
	11th to 20th	Night	46% 6% 11% 17% 18% 1	1654
	21st to end	Night	47% 6% 12% 18% 15% 3	1620
May	1st to 10th	Night	51% 6% 10% 20% 11% 2	1620
	11th to 20th	Night	44% 7% 9% 19% 19% 3	1593
	21st to end	Night	36% 8% 15% 25% 15% 3	1714
June	1st to 10th	Night	34% 7% 11% 27% 18% 3	1440
	11th to 20th	Night	35% 9% 14% 24% 16% 2	1440
	21st to end	Night	32% 7% 12% 28% 20% 1	1440
July	1st to 10th	Night	30% 7% 11% 28% 20% 4	1400
	11th to 20th	Night	29% 6% 12% 29% 20% 3	1392
	21st to end	Night	25% 9% 18% 24% 20% 4	1597
August	1st to 10th	Night	31% 7% 15% 31% 16% 1	1600
	11th to 20th	Night	31% 9% 12% 31% 14% 3	1598
	21st to end	Night	31% 7% 13% 26% 22% 2	1840
September	1st to 10th	Night	34% 8% 12% 26% 18% 2	1885
	11th to 20th	Night	31% 6% 10% 31% 21% 1	1980
	21st to end	Night	38% 7% 12% 27% 14% 1	1980
October	1st to 10th	Night	40% 6% 10% 24% 18% 1	2004
	11th to 20th	Night	50% 6% 11% 19% 13% 1	2160
	21st to end	Night	51% 6% 12% 21% 9% 1	2546
November	1st to 10th	Night	42% 6% 12% 24% 16% 1	2406
	11th to 20th	Night	51% 6% 14% 19% 9% 1	2470
	21st to end	Night	56% 6% 12% 18% 7%	2470
December	1st to 10th	Night	64% 6% 10% 13% 6%	2626
	11th to 20th	Night	60% 7% 13% 14% 6% 1	3320
	21st to end	Night	76% 5% 10% 6% 3	4884

Seeing (when cloud cover <= 80%)
Color Meanings: Too cloudy to forecast Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	23% 18% 36% 16% 6% 1	270
	11th to 20th	Day	24% 15% 42% 16% 3 1	255
	21st to end	Day	28% 15% 36% 16% 5% 1	264
February	1st to 10th	Day	25% 23% 36% 13% 3	240
	11th to 20th	Day	28% 20% 38% 12% 2	240
	21st to end	Day	25% 29% 33% 12% 1	198
March	1st to 10th	Day	24% 22% 34% 17% 2	258
	11th to 20th	Day	23% 11% 35% 23% 8%	420
	21st to end	Day	27% 13% 32% 20% 6% 1	462
April	1st to 10th	Day	20% 15% 34% 22% 9% 1	420
	11th to 20th	Day	25% 17% 32% 13% 8% 6%	412
	21st to end	Day	25% 12% 32% 18% 11% 2	420
May	1st to 10th	Day	24% 12% 30% 20% 10% 3	423
	11th to 20th	Day	19% 9% 32% 20% 13% 7%	447
	21st to end	Day	15% 7% 22% 27% 19% 9%	511
June	1st to 10th	Day	11% 6% 30% 32% 17% 4	630
	11th to 20th	Day	13% 7% 30% 28% 18% 4	630
	21st to end	Day	10% 8% 29% 30% 17% 6%	630
July	1st to 10th	Day	8% 5% 21% 29% 27% 12%	621
	11th to 20th	Day	11% 6% 26% 27% 22% 9%	618
	21st to end	Day	9% 4 23% 31% 24% 10%	568
August	1st to 10th	Day	9% 3 18% 29% 24% 17%	446
	11th to 20th	Day	13% 5% 16% 27% 27% 12%	439
	21st to end	Day	11% 3 23% 27% 25% 11%	495
September	1st to 10th	Day	14% 5% 21% 24% 22% 14%	450
	11th to 20th	Day	11% 3 18% 25% 28% 15%	450
	21st to end	Day	15% 10% 22% 23% 20% 9%	450
October	1st to 10th	Day	17% 9% 27% 24% 15% 8%	450
	11th to 20th	Day	20% 16% 29% 26% 8%	450
	21st to end	Day	31% 16% 31% 17% 5%	320
November	1st to 10th	Day	14% 18% 34% 24% 8% 1	270
	11th to 20th	Day	21% 22% 33% 18% 5% 1	270
	21st to end	Day	27% 18% 38% 13% 4	270
December	1st to 10th	Day	24% 21% 32% 15% 7% 1	191
	11th to 20th	Day	24% 20% 41% 11% 4	227
	21st to end	Day	27% 23% 33% 15% 2	297
January	1st to 10th	Night	22% 15% 35% 21% 5% 1	950
	11th to 20th	Night	26% 13% 34% 21% 5% 1	926
	21st to end	Night	19% 12% 36% 27% 6%	860
February	1st to 10th	Night	27% 12% 37% 22% 3	720
	11th to 20th	Night	23% 14% 35% 24% 4	703
	21st to end	Night	26% 13% 38% 21% 3	592
March	1st to 10th	Night	21% 13% 35% 25% 6% 1	720
	11th to 20th	Night	18% 16% 35% 25% 5% 1	720
	21st to end	Night	24% 11% 30% 27% 8% 1	792
April	1st to 10th	Night	18% 14% 35% 28% 4 1	590
	11th to 20th	Night	23% 9% 34% 26% 7% 2	533
	21st to end	Night	22% 7% 33% 29% 9%	540
May	1st to 10th	Night	20% 8% 31% 30% 10% 1	540
	11th to 20th	Night	22% 4 29% 31% 13% 1	531
	21st to end	Night	12% 4 24% 36% 21% 2	584
June	1st to 10th	Night	12% 4 23% 45% 16% 1	540
	11th to 20th	Night	12% 3 21% 44% 19%	540
	21st to end	Night	10% 3 21% 44% 21% 1	540
July	1st to 10th	Night	8% 3 15% 40% 30% 5%	525
	11th to 20th	Night	11% 3 16% 41% 25% 3	522
	21st to end	Night	4 2 19% 47% 26% 1	591
August	1st to 10th	Night	9% 2 15% 42% 30% 3	537
	11th to 20th	Night	9% 3 18% 38% 29% 4	534
	21st to end	Night	10% 1 17% 37% 32% 3	594
September	1st to 10th	Night	12% 5% 18% 35% 25% 5%	625
	11th to 20th	Night	12% 3 19% 32% 29% 5%	720
	21st to end	Night	14% 7% 21% 31% 22% 5%	720
October	1st to 10th	Night	15% 7% 27% 32% 16% 3	720
	11th to 20th	Night	22% 12% 29% 27% 8% 1	720
	21st to end	Night	26% 13% 33% 23% 6%	792
November	1st to 10th	Night	18% 11% 30% 27% 12% 1	740
	11th to 20th	Night	22% 14% 35% 21% 7% 1	760
	21st to end	Night	25% 13% 33% 23% 5%	760
December	1st to 10th	Night	23% 16% 35% 20% 6%	760
	11th to 20th	Night	23% 13% 38% 22% 4	907
	21st to end	Night	33% 12% 34% 20% 2	1045

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